1. Field of the Invention
The present invention relates generally to automatic transmissions for vehicles and, more particularly, to a five-speed automatic transmission for an automotive vehicle.
2. Description of the Related Art
A conventional automatic transmission includes a hydrodynamic torque converter to transfer engine torque from an engine crankshaft to a rotatable input member of the transmission through fluid-flow forces. The transmission also includes frictional units, typically referred to as clutches, which couple the rotatable input member to one or more members of a planetary gearset. Other frictional units, typically referred to as brakes, hold one or more members of the planetary gearset stationary during the flow of power. Such transmissions also typically provide for one or more planetary gearsets in order to provide various ratios of torque and to ensure that the available torque and the respective tractive power demand are matched to each other.
An example of an automatic transmission having a plurality of planetary gearsets is a ZF five-speed automatic transmission (5 HP 30) commercially available from Zahnradfabrik Friedrichshafen AG of Germany and found on the 1994 BMW 540i vehicles. The ZF five-speed automatic transmission includes a torque converter with a lock-up clutch, three drive clutch assemblies, three brake assemblies, and three planetary gearsets.
One disadvantage of the above ZF five-speed automatic transmission is that the arrangement of the planetary gearsets provides higher losses of efficiency in fifth gear because all three gearsets are loaded. Another disadvantage of the ZF five-speed automatic transmission is that it is more difficult with good planetary gearset design to provide a 0.75 overdrive ratio and good ratio spacing.